The transmission control device of the present invention comprises a main transmission and a sub transmission and has a first hydraulic switching device which is controlled by automatic operation for setting the shift range of the sub transmission and a second hydraulic switching device which controls the hydraulic pressure produced by the first hydraulic switching device, prohibits any speed change of the shift range, and controls other functions of the sub transmission. By the above-described structure, the transmission control device of the present invention is able to prohibit speed changes in the shift range and to control other functions of the sub transmission regardless of the automatic operation of the sub transmission, because the second hydraulic switching device can control the hydraulic pressure produced by the first hydraulic switching device even if the shift mechanism of the sub transmission is being operated by the first hydraulic switching device.